[BUG] Optimise unassigned shards iteration after allocator timeout

[imRishN]

Describe the bug

Currently, we would iterate over unassigned shards whenever we timeout pending batches after elapsed time. This might become expensive if shards take multiple iterations of reroute to get assigned.

Related PR comment - #14848 (comment)

Related component

Other

To Reproduce

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior

Optimise the behaviour by checking non executed shards at once and call the timeout method post that

Additional Details

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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
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[peternied]

[Triage - attendees 1 2]
@imRishN Thanks for creating this issue, could you make a pull request to address this or update the description to remove to make it clearer what should be done for another community member?